As shown in FIG. 1, a general hydraulic or pneumatic cylinder includes:
a cylindrical tube 1 that is opened at both ends thereof;
a cylinder rod 3 that linearly reciprocates in the tube 1 and includes a screw portion 2 formed at one end thereof;
a piston 4 that is screw-fastened to the screw portion 2 and is configured to partition the inside of the tube 1 into a large chamber and a small chamber to form a pressure when the cylinder rod 3 reciprocates in the tube;
a head cover 5 that prevents a hydraulic fluid from leaking through an opening formed at one side of the tube 1; and a cushion ring 6 that absorbs a mechanical shock of the piston 4 and the head cover 5 when the cylinder rod 3 is driven to extend to a maximum position.
In the drawings, non-explained reference numerals 7 and 8 denote a head rod and a cover end that allow the hydraulic or pneumatic cylinder to be rotatably mounted on an attachment of an excavator.
The fixed position of the piston of the hydraulic or pneumatic cylinder should be always constant in order to secure a stroke to completely perform a function of the hydraulic or pneumatic cylinder.
Meanwhile, the excavator is mainly used in an environment where the work conditions are poor. The piston of the hydraulic cylinder is exposed to an propulsive force, a severe vibration, and a shock by a high-pressure hydraulic fluid repeatedly applied to the piston.
For this reason, various loosening preventive mechanisms are used to prevent the piston 4 from being loosened from the cylinder rod 3. However, a loss of torque of the piston 4 with respect to the screw portion 2 of the cylinder rod 3 frequently occurs due to the shock repeatedly applied to the piston 4, and there is a high possibility that the internal parts of the hydraulic cylinder will be damaged due to the loosening of the piston 4.
Thus, hydraulic cylinder manufacturers employ the following methods as means for preventing the piston from being loosened from the cylinder rod after screw-fastening the piston to the cylinder rod:
1) A method for fixing the piston using a set screw, a key or a bolt;
2) A method for fixing the piston using a lock nut;
3) A method for fixing the piston using a nylon nut; and
4) A method for fixing the piston using a double nut of a left and right screw.
As shown in FIGS. 2(a), 3(a) and 3(b), a set screw for preventing the piston from being loosened from the cylinder rod in a hydraulic or pneumatic cylinder in accordance with the prior art is configured such that after a piston 4 is screw-fastened to a screw portion 2 of the cylinder rod 3, a screw hole 9 is axially formed in a mutual screw engagement portion of the cylinder rod 3 and the piston 4 and the set screw 10 is fasteningly engaged with the screw hole 9.
As shown in FIG. 2(b), a set screw for preventing the piston from being loosened from the cylinder rod in a hydraulic or pneumatic cylinder in accordance with the prior art is configured such that a piston 4 is screw-fastened to a first screw portion 3a of the cylinder rod 3 and a lock nut 11 formed integrally with the piston 4 is screw-fastened to a second screw portion 3b formed to have a diameter smaller than that of the first screw portion. In this case, a radial set screw 10 is fasteningly engaged with a screw hole penetratingly formed in the lock nut 11 in a radius direction of the lock nut 11 so as to prevent the piston 4 from being loosened from the cylinder rod.
As shown in FIG. 2(c), a set screw for preventing the piston from being loosened from the cylinder rod in a hydraulic or pneumatic cylinder in accordance with the prior art is configured such that a piston 4 is screw-fastened to a first screw portion 3a of the cylinder rod 3 and a lock nut 11 is screw-fastened to a second screw portion 3b formed to have a diameter smaller than that of the first screw portion. In this case, a set screw 10 is fasteningly engaged with a screw hole penetratingly formed in the lock nut 12 in a radius direction of the lock nut 12 so as to prevent the piston 4 from being loosened from the cylinder rod.
As described above, in the case where a torque of the set screw 10 additionally engaged with the screw hole to prevent the piston 4 from being loosened from the cylinder rod 3 is lost due to vibration applied to the piston 4, the set screw 10 is loosened or is separated from a fixed position thereof, and thus a function of completely preventing the loosening of the piston 4 cannot be expected.
For this reason, as shown in FIGS. 3(a) and 3(b), the piston 4 is screw-fastened to the screw portion 2 of the cylinder rod 3 and the set screw 10 is fasteningly engaged with the screw hole 9 in an axial or radial direction in such a manner as to caulk an inlet portion of the screw hole 9 so as to completely prevent the piston 4 from being loosened from the cylinder rod 3.
As shown in FIG. 3(a), in the case where a worker strikes four points around the inlet portion of the screw hole to caulk the inlet portion into a depth of 1-3 mm using a punching tool, a striking force or a caulking position of the punching tool is not uniform or a problem associated with stability of caulking modification is caused. For this reason, the set screw 10 engaged with the screw hole 9 to prevent the piston 4 from being loosened from the cylinder rod 3 may be separated from the screw hole.
As shown in FIG. 3(b), in the case where the set screw 10 engaged with the screw hole 9 to prevent the piston 4 from being loosened or separated from the cylinder rod 3 is loosened from the screw hole or separated from a fixed position thereof, the internal parts of a hydraulic cylinder may be damaged to degrade a performance thereof. In addition, a problem is involved in that the hydraulic cylinder may be damaged, leading to physical losses or casualties during the work.